1. Field of the Invention
This invention relates to a lead-free conductive paste composition, more particularly to a lead-free conductive paste composition used to form a back electrode of a solar cell.
2. Description of the Related Art
At present, a back electrode of a solar cell, particularly poly-silicon solar cells, is formed by screen printing and firing a conductive paste, e.g., an aluminum paste or a silver paste, on silicon wafers. Due to different thermal contraction between the silicon wafer and the conductive paste, a bowing phenomenon would likely occur after a high-temperature firing process.
Chinese patent application publication no. 1877864A discloses a conductive slurry composition and a method for producing the same. In this application, the conductive slurry composition includes aluminum powders, a modified organic binder, an inorganic binder, and glass metal powders. Since the aforesaid glass metal powders contain up to 50 wt % of lead oxide, the composition does comply with the EU RoHS Directive.
US patent application publication no. 2010/0059116 A1 discloses an aluminum paste comprising particulate aluminum and an organic vehicle. The particulate aluminum includes 30 to 90 wt % of spherical-shaped aluminum powder and 10 to 70 wt % of nodular-shaped (irregular-shaped) aluminum powder. Although, in this US patent application, the solar cell has reduced bowing phenomenon, due to the presence of the irregular-shaped aluminum powder, the packing density of a back electrode made from the aluminum paste is relatively loose, and the fill factor of the solar cell containing the back electrode is relatively low.